ipa-split: Don't split returns_twice functions [PR106923]

As discussed in the PR, returns_twice functions are rare/special beasts
that need special treatment in the cfg, and inside of their bodies
we don't know which part actually works the weird returns twice way
(either in the fork/vfork sense, or in the setjmp) and aren't updating
ab edges to reflect that.

I think easiest is just to never split these, like we already never
split noreturn or malloc functions.

2023-02-07  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/106923
	* ipa-split.cc (execute_split_functions): Don't split returns_twice
	functions.

	* gcc.dg/pr106923.c: New test.